1. Field of the Invention
This invention relates generally to road-construction and earth-moving vehicles, and relates more particularly to a non-contacting range sensing and control device for controlling the position of a grading implement relative to a datum.
2. Description of the Relevant Art
Motor graders, bulldozers, pavers and other road-construction and earth-moving vehicles are often called upon to produce a graded surface that follows a predefined datum. In areas of new construction, for example, the datum might be defined by a string line supported by stakes placed adjacent to the path to be graded. In areas of reconstruction, the datum might be defined by a preexisting curb or pavement surface.
Various devices have been used on earth-moving vehicles to position the grading implements to obtain the desired graded surfaces. The grading implements of these vehicles are usually positioned by hydraulic cylinders that are coupled to mechanisms that support the grading implements. A typical positioning device includes a datum sensing device that is mounted on the grading implement and that senses the position of the datum relative to the implement, and a control device that signals the hydraulic cylinders to reposition the implement accordingly.
One such datum sensing device is a string follower, used where the datum is defined by a string line that is suspended a constant distance above the desired graded surface. A sensor unit is attached to the side of the grading implement nearest the string line, and a pivotable wand extends from the sensor unit and touches the string line. The sensor is responsive to the rotational position of the wand as an indication of the position of the implement relative to the string datum. The wand is often spring loaded against the string line to ensure contact. One drawback to the use of a string follower is that if the spring force is excessive, or if the string line is loosely strung, then the spring force of the wand can displace the string line from its intended position and thereby introduce grading errors. Another drawback is that the operator must stop grading and get out of the cab in order set the wand onto the string line. Another drawback is that if the wand falls off of the string line, then the sensor indicates a large positional error and tries to correct the position of the implement accordingly, thus causing gouges or other discontinuities in the graded surface. Still another drawback is that the wand mechanism typically has limited adjustability, which restricts the location of the string line relative to the desired surface.
Other datum sensing devices include wheels and skids, which are useful where the datum is defined by a pre-existing curb or previously graded surface. A sensor unit is usually mounted to the side or rear of the grading implement, and a projecting arm pivots downward to place the wheel or skid on the datum surface. The sensor unit responds to the rotational position of the arm as a measure of the position of the implement relative to the datum surface. One major drawback to the use of wheels and skids is that they are typically designed for forward movement of the vehicle, so that in order to allow the vehicle to back up, the wheel or skid must be lifted up. If the vehicle backs up without lifting such a wheel or skid, then the mounting mechanism may bend or break off.